You can put a stripe of reflective tape across the spinner and read it like a 2 blade prop. In 13 years I can't ever remember taching a glow powered D/F. I did tach an Electric D/F to compare 2 different motors. I set my inflight needle by going to peak (by ear) and then back of a little, looking for a good smoke trail. I always use a fuel with 23% castor/synthetic oil blend.